Not familar with the SAG rules, do they state something like "his last films were big hits, so top bill him"?

I'm not sure what the exact rule was. It might be if you won an Academy Award, in which case he didn't have to receive top billing. However, a good example is Star Trek: Generations. SAG Rules would have mandated that Whoopi Goldberg be listed first (as opposed to William Shatner and Patrick Stewart). Since Whoopi didn't think that was fair, she agreed to not be listed at all in order to comply with the rules.